1. Field of the Invention
The present invention relates to an electro-photographic or an electrostatic recording system, and specifically, relates to a method for transferring a developed image from an image carrier to a transfer material or intermediate transfer member.
2. Description of the Related Art
As shown in FIG. 5, a conventional image electro-photographic system such as a copy machine or laser printer typically includes an electro-photographic photoconductor (photoconductive drum) 1, which is an image carrier, and an electrifier 2 for electrifying the photoconductive drum 1. Other components of such a conventional electro-photographic system include an exposure unit 3 and developing devices 4Y, 5M, 6C, and 7K for color components of yellow (Y), magenta (M), cyan (C), and black (Bk), respectively. Each of these developing devices contains a magnetic material known as a developer for carrying toner.
As shown in FIG. 5, the system works by using the photoconductive drum 1 to form toner images of respective CMYK colors. For each drum rotation, toner image is electrostatically transferred onto a belt-shaped transfer medium (transfer belt 9) opposite the photoconductive drum 1. For full color images, four colors are superimposed on the intermediate transfer belt 9 one upon another and are then transferred to the transfer material 20 to form the color image.
As shown in FIG. 6, during the developing process, toner image is developed by controlling the potential on the photoconductive drum 1, which is uniformly electrified by the electrifier 2 to a drum potential Vd=+450 V. Two areas of the toner image are formed. First, a non-image portion, which is a region where no toner image is developed, is exposed by the exposure unit 3 (latent forming means) to de-staticize this area to a potential Vl=+100 V. The second area is the image area left unexposed to remain at a potential of Vd=+450 V to form an electrostatic latent image.
In FIG. 5, a development potential of Vdc=+250 V is applied to the developer device 4Y (FIG. 6) when it reaches a designated developing position of the photoconductive drum 1. As a result, development is performed on the image portion using negative toner from the developer device 4Y. Development is based both on a factor known as tribo, which is the tribo-electric charge of the toner per unit mass, and a difference in potential between Vd, the potential on the drum surface for the image portion and Vdc, the bias potential applied to the developer device 4Y, that is, Vcont=Vd−Vdc as shown in FIG. 2.
The tribo can vary depending on environmental conditions such as the absolute moisture content. When equal amounts of toner for a color are developed on the photoconductive drum 1, a smaller Vcont is needed when the tribo is low, while a larger one is needed when the tribo is high.
Even if the environmental conditions are equal, the values of tribo vary for different toner colors, and therefore, proper values of Vcont are required for each of the colors in order to develop a proper amount of toner.
The potential difference Vback between the development potential Vdc and the exposure potential Vl as shown in FIG. 6, is a potential difference for preventing a magnetic carrier from adhering to the drum and/or for inhibiting toner from adhering to a non-image region. Regarding this potential difference, in many cases, there is no problem even if the potential difference is constant irrespective of environmental conditions such as the absolute moisture content and the color.
Accordingly, when attempting to develop a proper amount of toner on the photoconductive drum 1 in the normal development, it is necessary to adjust values of Vcont to proper ones for each environment and each of the colors. In this case, the drum potential Vd is fixed, and then the exposure potential Vl is changed by an adjustment amount of the Vcont by adjusting the exposure amount of the exposure unit 3. Then, the development potential Vdc is changed by the changing amount of the exposure potential Vl, whereby Vback is adjusted so as not to change.
For example, when attempting to adjust Vcont from 200 V (FIG. 6) to 150 V, as shown in FIG. 7, the drum potential Vd is left as it is (i.e., +450 V), and then the exposure potential V1 is changed to +150 V by adjusting the exposure amount of the exposure unit 3 so that the exposure potential Vl changes by the Vcont adjustment amount 50 V. Then, the development potential Vdc is changed to +300 V by likewise changing it by 50 V. As a result, Vcont is adjusted from 200 V to 150 Vback, with Vback left as it is (i.e., 150 V).
In this manner, the adjustment of Vcont allows a satisfactory developed image to be formed. For the optimization of the developing performance of toner of respective colors, the values of Vl can be set for each of the colors.
A transfer bias is applied to a transfer member 15, which makes contact with the intermediate transfer belt 9 on the rear surface side in order to transfer images from the photoconductive drum 1. This also allows the reduction in a power supply cost by a low output. The transfer member 15 is usually a contact rotary type roller, hereinafter referred to as a “primary transfer roller 15.”
Specifically, a charge opposite the toner polarity is imparted from the primary transfer roller 15 to a primary transfer region to toner image from the photoconductive drum 1 to the intermediate transfer belt 9 in an electrostatic manner.
During application of a transfer bias to the primary transfer roller 15 to pass a current having an opposite polarity with respect to toner, if the potential difference between the exposure potential Vl and primary transfer roller potential Vtr as shown in FIG. 2 is high, the current flows easily, while current flow is difficult when the potential difference is low.
Even with equal potential differences, if the resistance of the primary transfer roller 15 is higher, the current flow remains difficult.
Specifically, current flow through the primary transfer roller 15 varies depending on the exposure potential of the non-image portion of the photoconductive drum 1, and the resistance value of the primary transfer roller 15. Therefore, if the current flowing from the primary transfer roller 15 can be properly controlled, a developed toner image would be primarily transferred in a proper manner.
The resistance value of the primary transfer roller 15 is adjusted to a value on the order of 106 to 1010 [Ω]. As shown in FIG. 8, a conventional transfer roller has an elastic layer 15b formed on the outside of an electrically conductive core metal 15a. The elastic layer 15b is provided with electrical conductivity. The transfer roller 15 is broadly classified into two types in accordance with the method for imparting an electrical conductivity.
Out of these two types of transfer rollers, one type, which has an electronic conductivity, is provided with the elastic layer 15b shown in FIG. 8, the elastic layer 15b being formed by dispersing an electrically conductive filler thereinto. Examples include an EPDM (ethylene-propylene-diene copolymer) roller and a urethane roller each of which is formed by dispersing an electrically conductive filler, such as carbon or a metal oxide, thereinto.
The other type of transfer roller 15, which has an ionic electric conductivity, comprises an ionic electric conductive material in the elastic layer 15b. Examples include a roller formed by providing an electrical conductivity to its material itself, such as urethane, and a roller formed by dispersing a surface-active agent into the elastic layer 15b. 
The resistance of the transfer roller 15 is prone to vary depending on the temperature and humidity in the apparatus and energization time. As a result, once a resistance variation of the primary transfer roller 15 have occurred, it is impossible to impart a proper charge to the above-described primary transfer region. This causes apprehension that an occurrence of primary transfer defects might be induced.
Japanese Patent Laid-Open No. 10-133495 discloses a method for setting a transfer bias based on the temperature and humidity results but the amount of resistance variation due to energization during the image forming process cannot be predicted.
Also, Japanese Patent Laid-Open No. 5-6112 discloses a method that, in a pre-rotation process directly before an image forming (imaging) process, uses the transfer voltage at the time when a transfer voltage applied to the primary transfer roller 15 is increased step by step and a desired transfer current has been reached. However, if there is a large difference between a transfer voltage initially provided and an optimum transfer voltage, it will take much time to reach the desired voltage, that is, the image forming process (imaging process) will not readily start. Thus, this method has a problem in that much time elapses before getting down to the image forming operation.
To simplify circuitry, a constant voltage is used as a transfer bias. Specifically, in order to prevent the primary transfer defects caused by resistance variation of the primary transfer roller 15 during pre-rotation, another method uses the relationship between the voltage applied to the primary transfer roller 15 and the current flowing through the primary transfer region to obtain resistance so that the primary transfer voltage applied to the primary transfer roller 15 is properly controlled.
To detect resistance during the pre-rotation process, respective current values for applied predetermined voltages are detected, and based on these plural voltage and current values, a voltage-current characteristic function is specified to determine the resistance characteristic of the transfer roller.
In the present description, the “pre-rotation” refers to a time period for which each image forming means operates within the time period between the time point when a signal from outside is transmitted to the image forming apparatus and the time point when the signal arrives at the position where a first developed image is transferred, i.e., a transfer portion.
It is preferable that such a detecting operation with respect to the resistance value of the transfer roller be performed at a time during non-image forming operation, that is, when no image formation is conducted. Accordingly, here, the resistance value detecting operation is performed at a pre-rotation process. However, with the time when image formation by the transfer roller is conducted being assumed as a non-image formation time, the resistance value detecting operation may be performed even during an image forming process except during transfer process.
The resistance value of the transfer roller for transfer control is detected when the surface potential of the photoconductive drum equals a non-image portion potential (exposed portion potential Vl).
However, substantial time can also elapse before imaging actually occurs. When a plurality of predetermined voltages (e.g., voltages at three levels)are applied to the primary transfer roller 15 during pre-rotation and when the surface potential of the photoconductive drum 1 equals an exposure potential of each of the colors, the primary transfer roller 15 is rotated at least three times per color for a total of twelve rotations for four colors since the primary transfer roller 15 must be rotated once per unit level of voltage. This causes substantial time to elapse during the pre-rotation stage between the start of operation and the formation of a first image on the transfer material 20, so that productivity is reduced.